People once believed Earth was only several thousand years old. Today there’s evidence that some rocks are billions of years old. Scientists can determine the relative age of a rock by comparing it with another rock. The relative age of a rock is based on two ideas. One is the principle of original horizontality. This idea explains that sedimentary rocks form in horizontal layers. The second idea is superposition. This idea explains that in a series of rock layers, the bottom layer is the oldest, and the top layer is the youngest. Using these ideas, scientists put together a geologic column listing rock layers from oldest to youngest. This column is miles thick. It was also determined that to build up rock layers as thick as the geologic column, it would take millions of years.